The present invention relates to a calender, comprising a calender frame and a stack of at least four rolls mounted on the frame and consisting of four rolls arranged vertically one above the other. In the stack, each roll placed one above the other can be situated in nip contact, i.e., nip-defining relationship, with an adjacent roll so as to calender the paper web or equivalent in the nips thus formed.
Strict requirements are imposed on a modern calender vis-a-vis loading capacity and mode of calendering so as to provide the desired properties of the quality of paper. For example, over the course of several years, the standard newsprint has developed into several different quality categories, each of which requires a different mode of calendering of its own. Specific requirements have been imposed on the calender especially by these different quality categories, and it has not been possible to meet these requirements by means of earlier prior art calender arrangements. This factor has also contributed to making soft calenders with two nips more common. In such an arrangement, both nips of the calender are independent in such a calender with two nips so that the loads and the temperatures of rolls can be regulated as desired. Drawbacks of the soft calender with two nips have been the high cost of the arrangement and that a calender which consists of separate nips occupies a large amount of space in the machine direction.
Furthermore, environmental requirements have become stricter recently, which has had the consequence that paper is recycled to an ever greater extent by producing recycled stock out of reclaimed paper. The introduction of recycled stock and, further, the change over to ever thinner grammages in paper grades set their own strict requirements on the calender.